1. Field of the Invention
The present invention relates to the field of zoom techniques for imaging. Specifically the present invention relates to the field of tracking an object with a varying position of a zoom area in an ECT nuclear medicine imaging camera system providing a varying field of view.
2. Prior Art
Gamma detection cameras, also called gamma cameras, are used for medical imaging of particular body tissues, organs, or bone that may otherwise not be available for examination. In a typical medical camera of this sort, a special gamma ray emitting radiopharmaceutical is injected into the body and accumulates into the area of interest within the patient. The patient is then placed within the medical camera's imaging surface. As is well known, the radiopharmaceutical emits gamma rays which are then detected by the gamma camera as a series of photon emissions from a specialized crystal layer. Before the gamma rays reach the crystal they travel through a collimator layer which allows only those gamma rays which travel perpendicular to the collimator's orientation. A matrix of photomultiplier tubes is optically coupled to the crystal layer to receive the photon bursts, or scintillations, within the crystal layer and converts these photon bursts into electrical signals indicating a spatial coordinate of the gamma radiation. By using computers and other processing equipment to store and display the signals from the gamma camera, an image of the organ containing the radiopharmaceutical can be obtained and displayed for examination and diagnosis. When the gamma camera rotates around the patient and transverse images are reconstructed the system is called an emission computed tomography or ECT system. The surface of the gamma camera which receives the gamma rays from the patient is called the imaging surface. Since the collimator of the gamma camera is the first or outermost layer of the gamma camera, the collimator surface is commonly referred to as the imaging surface of the camera.
In practice, an object (patient) is placed horizontally into a central location while a gamma camera rotates (transaxial rotation) around a predetermined portion of the object to collect a number of images about that portion. This "ECT" rotation is orthogonal to the cranial-caudal axis of the patient. The acquired images are reconstructed into cross-sectional slices or images of the patient or object organ (or bone) at the predetermined location along the cranial-caudal axis of the object. For an total body scan, the gamma camera moves along the patient. As the camera surface translates, it collects the radiated gamma rays from the radiopharmaceutical. To obtain best quality images, it is desired to place the collimator surface as close as possible to the patient's outer surface. It is universally understood that when the collimator to patient distance is minimized better image resolution develops.
The resolution in the final reconstructed cross-sectional images is limited by the detector, the collimator, the distance between the collimator and the patient and by several other factors. One limitation is the sampling or size of the computer picture elements (pixels). Resolution improves if the size of the pixels is reduced (increased sampling). The gamma camera computer processor (image processor) processes a finite number of pixels to create an image. It would be advantageous to reduce the pixel size to improve image quality. The number of display pixels on the display screen, is distributed throughout the field of view of the imaging surface of a scanning camera. Therefore, a small object located within a large field of view of the imaging surface may appear relatively small on the display screen.
Recent ECT scanning cameras contain large rectangular field of views, typically 20 inches by 15 inches in dimension. This value represents the area of the imaging surface (collimator layer) of an ECT scanning camera. This is enough to accommodate the width of a human chest for a total body scan. However, if the particular organ or tissue of interest is only on the order of 4 to 5 inches wide, its representative image on the display screen will be small relative to the large field of view of the imaging surface. To this extent, relevant features of the small object may become obscured, hidden or difficult to interpret using a large field of view camera. To solve this problem, prior an systems have created a zoom region over a portion of the total field of view of the imaging surface. This zoom region has a zoom field of view that is smaller than the total field of view. The zoom region may be thought of as the active scanning region of the imaging surface; while those areas of the imaging surface not in the zoom region are passive in that they are temporarily not sending data to be imaged. For instance, for a square ECT imaging surface 20 inches by 15 inches, the zoom region may become 10 inches by 10 inches in area. The smaller zoom region (area) would then totally exist within the field of view of the imaging surface of the camera. All of the pixels available to the scanning camera would then be employed on this zoom region. The resulting image display of the small object within this zoom region would be larger than if the small object was imaged by the entire field of view.
The image discrimination resolution of a gamma camera is typically about 3-4 min. The resolution of the collimator is about 6 to 20 mm depending on the proximity of the collimator to the object. Given a 20 inch by 20 inch field of view there are approximately 508 mm by 508 mm in the field of view. Using an ECT study of 64 by 64 pixels, each pixel is approximately 9 mm by 9 mm in dimension. If the field of view of the imaging surface was reduced to 300 mm then the dimension of the pixel is now 5 mm and capable of finer discrimination of the object and the radiopharmaceutical. Typical resolution of the gamma camera is from 10-15 mm thus pixels are approximately 5-7.5 mm in dimension. Therefore, by reducing the field of view of the gamma camera, the resultant pixel dimension reduces in size and the imaging camera is capable of discerning more image detail. By utilizing the above procedure, a small object or organ can be displayed with excellent resolution by relatively minor modifications to an existing ECT system.
FIG. 1 illustrates the zoom region implemented in the prior art as discussed above. FIG. 1 is a frontal view along the long axis of the ECT scanning camera in one dimension. The side view of one position of the imaging surface 200 of an ECT camera is illustrated. A relatively small object 59 is also shown. In a well known manner, this ECT camera may rotate about a center of rotation 10 along a gantry structure (not shown) during the ECT scan; this rotation is called ECT movement. The object of interest 59 is located about the center of rotation 10 of the ECT system. The fixed location zoom region 80 of imaging surface 200 is also shown as the shaded portion. By locating the object of interest in and around the center of rotation 10, when the imaging surface rotates to a new position 200' (shown by the dashed lines), the zoom region 80 of the imaging surface remains in exactly the same relative fixed location with respect to the imaging surface. As is seen within position 200' the zoom region 80 is still located within the center of imaging surface 200'.
Systems of the prior art usually locate the object of interest close to the center of rotation of the ECT system because the zoom region 80 of the prior art systems occupies a fixed location with respect to the dimensions of the imaging surface 200. Therefore, as the imaging surface 200 moves through position 200', the zoom region 80 remains situated within the same relative position with respect to the imaging surface. However, if the object 59 was located off of the center of rotation 10, the object would not remain within the fixed zoom region 80 throughout the ECT scan movement because the imaging surface rotates around the center of rotation and not around the object. This is disadvantageous because in many ECT scanning operations it is desirable to place the object of interest as close to the imaging surface as possible to increase the quality of the resultant image. By placing the object 59 close to the imaging surface the object is inherently placed off of the center of rotation of the ECT camera.
Also, some scanning camera systems employ dual camera imaging surfaces that may be placed orthogonal to each other. It would be desirable to place the object 59 as close as possible to each imaging surface for high quality images. However, it is impossible to place the object 59 close to both orthogonal imaging surfaces while maintaining the object 59 within the center of rotation 10 of both imaging surface. It is desirable, rather, to place the object in the inside corner of the two imaging surfaces. Therefore, what is needed is a system capable of defining a zoom region corresponding to the initial placement of an object of interest within the field of view of an imaging surface then allow the relative position of that zoom region with respect to the imaging surface to move during ECT motion of the imaging surface. This would allow an object to be placed in the comer of two orthogonal imaging surfaces because the zoom regions (relative to the imaging surface) of each imaging surface could vary during the ECT motion of the imaging surface. The present invention allows for such an advantageous capability.
Therefore, it is an object of the present invention to allow a zoom region within an imaging surface of an ECT camera to be initially defined by the position of an object of interest (that may be off of the center of rotation of the imaging surface), then to move the zoom region relative to the imaging surface as the imaging surface undergoes ECT motion around the object of interest. In so performing the above, it is an object of the present invention to image a point of interest located on an object of interest where the point of interest is not located on the center of rotation of the ECT camera system. It is further an object of the present invention to advantageously utilize the above capability within the environment of a dual scanning camera ECT system. Therefore, the zoomed region will track a small point of interest or an organ of interest that is offset from the center of rotation of the ECT camera system. These and other objects of the present invention not specifically mentioned will be further discussed in the detailed discussion of the present invention.